1. Field of Invention
The present invention relates to production of specific types of light with a high degree of fourth-order coherence that can be used in certain systems. In particular, the present invention is directed to methods for creating entangled photons and source for producing those entangled photons. More specifically, the present invention produces polarization-entangled photons. The entangled photons may be beneficially used in communications, computing and other applications.
2. Description of Related Art
In the field of quantum information, quantum optics, quantum cryptography, and quantum communications, etc., there exists a need to generate entangled photon pairs. The entangled photon pairs are described by an inseparable wave equation such that if a measurement is performed on one photon, its other twin photon's state is completely determined. Such photon pairs are also described as having a high degree of fourth-order coherence. Fourth-order coherence occurs when two-photons that are highly correlated as a result of quantum entanglement are brought together and observed in coincidence; the quantum interference between the two particles will only be visible when viewed using time-coincident techniques. Without the fourth-order coherence, coherent interference effects are not observed. The problem up to now is that these sources of entangled photons require large, expensive and power-intensive Ar-ion lasers to generate light in the UV to pump a non-linear crystal, such as beta barium borate (BBC)), to produce spontaneous parametric down conversion (SPDC). The SPDC process generates a pair of photons (the signal and idler) whose momentum and energy sum up to equal the initial pump photon.
One such SPDC process is illustrated in FIG. 1. An incoming beam 101 is incident on the crystal 110 to produce pairs of photons 120. The photons produced form a spectrum with wavelengths centered around twice the wavelength of the incoming beam and form conical emissions based on the properties of the crystal. In the process illustrated, the polarizations of the produced photons are opposite to that of the incoming beam. Additionally, if two crystals are used, where one crystal is rotated 90° from the other, then polarization-entangled photons can be produced. Such a system is illustrated in FIG. 2. In that example, the incoming beam 201 is incident on a first crystal 210 coupled with a second crystal 212. The incident 201 is polarized linearly at an angle of 45 degrees with respect to one of the crystals 210 and 212. In this way a photon from the pump beam 201 has a equal probability of down-converting in either crystal 210 or 212. Since the crystals 210 and 212 are thin and located next to each other, the indistinguishability of which crystal actually provided the downconversion produces the polarization entanglement. The photons produced have polarizations oriented at 90° with each other, the optical axis of a first 222 lies in a vertical plane and the optical axis of the second 220 in a horizontal plane. Such systems are discussed, for example, in “Ultrabright source of polarization-entangled photons”, P. G. Kwiat et al., Physical Review A, Vol. 60, No. 2, pp. 773-6, 1999.
However, in the prior art, such systems typically take up a great deal of space on a laboratory optical table, weigh several hundreds of pounds and consume tens of kilowatts of electrical power and require cooling water. Other problems of such systems are their rather inefficient conversion of electricity to usable QE photons. The power input to a 10 Watt Ar-ion laser is about 25 kW, while the final output SPDC photons is on the order of thousands of photons. A single photon per second has a power of about 1E-19 W. Thus, the power efficiency of conventional SPDC systems is only about 1E-23.
Prior art systems typically use a large gas Ar-ion laser to pump an externally mounted nonlinear crystal producing two QE photons at about 702 nm from a single 351 nm pump photon. Other devices, such as a high finesse optical cavity have been used to enhance the efficiency by a factor of 20 to 100. One main problem with the prior art is that the systems are bulky and very power intensive. Such prior art systems do not work well as a standard component in telecom, computer networking, field portable devices, or miniature nano-technology devices.
Recently, however, the use of blue 400 nm diode lasers to pump nonlinear crystals external to the pump laser has been described. See, for example, “Entangled photon apparatus for the undergraduate laboratory” D. Dehlinger et al., Am. J. Phys., vol. 70, No. 9, pp. 898-902, 2002. However, such a system still requires large macro-size optical components, is sensitive to optical alignment and is not portable.
Thus, there is a need in the prior art for a system or reduced size to a level compatible with nanotechnology or at least micro-scale structures of cubic millimeter size. There is also a need for a system that may be produced at reduced cost and complexity by integrating everything onto a single monolithic component. There is also a need for a system that that is more robust and less sensitive to optical alignment than the prior art systems, so that it may be used in the field such as in telecom components or portable devices. There is also a need for a system that will consume very little electrical power (mW). There is also a need for a system that will improve efficiency by producing approximately thousands of entangled photons for mW of electrical input power; thus increasing the efficiency by about a million times over the conventional techniques.